Pumping oil from geological structures requires a pumping unit. The pumping unit oscillates a sucker rod string in an essentially vertical direction in repetitive upstrokes and downstrokes. The unit is driven by a relatively high-speed motor driving reduction gears which develop the reciprocating action. However, the resistance to the pumping presented by the down-hole pump, the weight of the sucker rod string, and the weight of the oil is quite different on the upstroke than the resistance presented on the downstroke. Since the pumping unit overcomes this resistance through the development of torque, the differences in resistances develop differences in torque on the upstroke and downstroke.
The differences in upstroke and downstoke torque cause imbalances in the pumping unit mechanism and result in excessive wear and tear on the various components of the unit and premature failure of the unit. These imbalances also waste precious energy in overcoming the unequal resistances. Ideally, the unit would develop the same torque on the upstroke as on the downstroke.
Known systems have tried to overcome the imbalance problem in primarily two ways. Some systems use counterbalance weights to try to equalize the imbalance in upstroke and downstroke. These systems are static in that they make no account of changes in upstroke and downstroke torque; they achieve an ideal balance at only a single relative difference in upstroke and downstroke torque. Further, these systems are highly inefficient in that the driving motor and mechanism must maintain the rotary motion of the heavy counterbalance. These systems do, however, provide an advantage in that they are very simple.
The other major type of system that tries to overcome the imbalance problem is the air-balance system. These systems provide a pneumatic cushion that differs on the upstroke and downstroke so that the torque in the two directions is more nearly equal. Early air-balance systems used a Murphy switch to maintain a minimum amount of pressure in the system. If air pressure within the system got too low and reached the lower trip setpoint, compressed air from an air compressor or air receiver was pumped into the system until system pressure reached the upper trip setpoint of the Murphy switch at which point the air pumping was stopped.
Such a system suffered from the drawback that it made no account for effects on the various components of the pumping unit. It simply maintained pressure within the set band, regardless of effects on the motor driving the pumping unit, for example.
Other known systems attacked this drawback by providing a means for monitoring system motor rpm. Such a system is made by Nabla of Midland, Tex. and is described in "World Oil, " May, 1989. This system retained the Murphy switch but set the low and high pressure settings further apart as a backup should the monitor fail to activate or stop the compressor. The monitor of this system converts rpm data to motor output torque from motor torque/speed performance data. Such a system suffers a drawback in that it derives motor torque empirically, making no account for motors that vary from the performance data, either from motor to motor, or as a result of aging in the motor.
Thus, there remains a need for an air-balance control system for a pumping unit that measures parameters of an operating pump unit directly and utilizes those measured parameters to dynamically balance the operation of the system. Such a system should operate on actual operating conditions and not rely on performance data.